Field of the Invention
This invention relates to polyorganosiloxane foams. More particularly, this invention relates to one-part, nonslumping foamable polyorganosiloxane compositions that include liquified blowing agents. In the presence of atmospheric moisture and atmospheric pressure, the compositions form foams that cure with a minimum of collapse to yield useful, low density products.
It is known to prepare foams by introducing a gaseous blowing agent such as compressed air into one-part, moisture curable polyorganosiloxane compositions that are typically employed as room temperature vulcanizable (RTV) sealants. A characteristic of known RTV compositions is the long time period required for these compositions to cure to the extent that the foam becomes self supporting. One way to maintain the cellular structure of the initially produced foam during curing is to place a flowable RTV polyorganosiloxane composition in a vacuum chamber for a period of time sufficient to obtain a self-supporting foam. This technique is taught by Modic and Boudreau in U.S. Pat. No. 4,368,279, which issued on Jan. 11, 1983.
While the use of vacuum during curing may be practical for machine made foams such as slab stock and molded foams, this technique cannot be used when the foam is formed within a cavity of a building or other locations where it is not feasible to maintain the foam under vacuum during curing. In addition, the necessity of mixing in a gaseous blowing agent at the time the foam is formed requires that mixing and aereating equipment be transported to the location where the foam is to be installed. For some applications, particularly those requiring relatively small amounts of foam at remote locations, such equipment would not be practical. In these instances it would be considerably more convenient to employ a one-component foamable composition, including a blowing agent, packaged in a container that can be easily transported to the application site and which is capable of repeatedly dispensing the foamable composition without the need for additional processing steps or ingredients.
A second method for reducing the collapse of partially cured foams prepared using RTV polyorganosiloxane compositions is to incorporate fillers such as silica and calcium carbonate. Filled RTV compositions have been packaged in 2-compartment aerosol cans together with a compressed gas that supplies the pressure required to expel the composition from the can in addition to forming the cellular structure of the foam. One package foamable compositions are described in U.S. Pat. No. 4,229,548, which issued to Sattleger et al. on Oct. 21, 1980, and in German published application No. 2,911,971, which was published on Oct. 9, 1980. The cured foams are typically of relatively poor quality and characterized by average cell sizes larger than 2 mm, densities from 0.48 to 0.81 g./cc and relatively low foam height due to drainage of uncured or partially cured liquid from the cellular structure of the foam during the curing process. The need to minimize collapse of partially cured foams by the use of vacuum, by heating to accelerate curing, by the addition of large amounts of fillers or by other means requiring additional processing steps may more than offset the advantages achieved by using foamable compositions packaged in portable containers such as aerosol cans.
A disadvantage of some of the uncured foamed compositions disclosed in the Modic and Boudreau patent discussed hereinbefore is that these compositions will flow for a distance of several centimeters when placed on a vertical or sloping surface. This phenomenon is referred to as "slumping." This problem typically does not occur in highly filled compositions containing a nonliquifiable blowing agent such as air.
The advantage of liquifiable blowing agents is their ability to function as solvents while in liquified form in an aerosol container. This solvent action permits use of high molecular weight polymers that would otherwise be too viscous for use in compositions intended for dispensing from aerosol containers. High molecular weight polymers are desirable because their high viscosity minimizes drainage of liquid materials from partially cured foams.
The present invention is based on the discovery that two major disadvantages of prior art RTV foamable polyorganosiloxane compositions, namely relatively high density and foam collapse, can be reduced to acceptable levels and the slumping of uncured compositions can be substantially reduced by including in the composition a relatively small amount of filler, a liquified blowing agent and a class of diorganosiloxane graft copolymers described hereinafter. The resultant cured foams exhibit a desirable combination of low density, small average cell size and high cell concentration.